1. Field of the Invention
The present invention relates to a scanning probe microscope capable of acquiring surface data of a sample by detecting an interaction between probe and sample while two-dimensionally scanning the surface of the sample.
2. Description of the Related Art
FIG. 10 shows a general scanning probe microscope (to be called as SPM hereinafter), in which the surface data of a sample 1 is acquired by detecting an interaction between the sample 1 and a probe 2 while two-dimensionally scanning the surface of the sample 1 by the probe 2. If the surface data is recess-and-projection data, the recess-and-projection state of the sample surface is displayed.
A sample stage 3, on which the sample 1 is placed, is fixed on the upper end surface of a cylindrical piezoelectric unit which can be finely moved in the Z direction (up and down direction), and X and Y directions (two directions crossing normal to each other in a horizontal plane which is perpendicular to the Z direction). The proximal end of the cylindrical piezoelectric unit 4 is fixed to a microscope body 5.
The cylindrical piezoelectric unit 4 consists of a cylindrical piezoelectric element and a plurality of pairs of electrodes each pair sandwiching the lateral surface of the piezoelectric element from the inner and outer sides. The piezoelectric element has characteristics in which the mechanical length varies in accordance with the magnitude of an applied voltage, and therefore the cylindrical piezoelectric unit 4 can be displaced in the Z, X or Y direction by selecting an electrode pair to which a voltage is applied, and adjusting the magnitude of the applied voltage. Jap. Pat. Appln. KOKAI Publication No. 63-236992 discloses a fine movement mechanism of the piezoelectric element which can move finely in the three-dimensional directions.
In the SPM shown in FIG. 10, the position of the probe in the Z direction is adjusted as a probe control unit 6 controls the voltage value of a scanning signal Sz applied to the cylindrical piezoelectric unit 4, the displacement of the probe in the X direction is adjusted as a feedback drive circuit 7 controls the voltage value of a drive signal Sz applied to the cylindrical piezoelectric unit 4, and the displacement of the probe in the Y direction is adjusted as a feedback drive circuit 8 controls the voltage value of a drive signal Dy applied to the cylindrical piezoelectric unit 4.
The instruction for scanning in the X and Y directions is supplied from a microcomputer 9 to the feedback drive circuits 7 and 8. In the case where the probe is moved in the X direction, the microcomputer 9 sends a scanning instruction signal D1, which linearly increases or decreases at a predetermined angle, to the feedback drive circuit 7. In the case where the probe is moved in the Y direction, the microcomputer 9 sends a scanning instruction signal D2, which linearly increases or decreases at a predetermined angle, to the feedback drive circuit 8.
It should be noted here that a general piezoelectric element has a hysteretic relationship between an applied voltage and a change in the mechanical length, as shown in FIG. 12. Therefore, conventionally, the feedback control is carried out by the feedback drive circuits 7 and 8. More specifically, in order to detect the displacement of the sample stage 3, an X direction displacement sensor 10 and a Y direction displacement sensor 11 for detecting the displacements in the X and Y directions are provided as shown in FIG. 11. The displacement sensor shown in FIG. 11 is of an optical reflection type in which sensor light is irradiated on the lateral surface of the sample stage 3 by using optical fibers 10a and 11a, and the displacement is detected from the intensity of reflection.
The feedback drive circuit 7 and 8 receive displacement detection signals from the X-directional displacement sensor 10 and the Y-directional displacement sensor 11 at a predetermined cycle, and drive signals Dx and Dy are subjected to the feedback control so that the waveform of the displacement detection signal becomes the same as that of the scan instruction signal.
Meanwhile, the Z-directional distance between the sample 1 and the probe 2 is detected by a probe displacement detection unit 15 at all times. For example, the Z-directional displacement of the probe 2 is detected by the known optical lever method, and a displacement detection signal Pz is input to the probe control unit 6. The probe control unit 6 controls the voltage value of a scan signal Sz so that the probe is at a target Z position when the displacement detection signal Pz varies. Further, in response to a request from the microcomputer 9, a Z position control data D3 (corresponding to scan signal Sz) formed by the probe control unit 6 is supplied to the microcomputer 9, and image data D4 obtained by converting the Z position control data D3 into sample surface data is sent from the microcomputer 9 to the host computer 16. Thus, the sample surface image is displaced on the screen of the host computer 16.
Thus, the distortion of the sample surface data image which is caused by the hysteretic relationship between the applied voltage to the piezoelectric unit and the displacement is removed, and an image having a high linearity is obtained by a host computer.
The conventional SPM described above has a high resolution of displacement of the piezoelectric unit 4 of about 0.01 nm; however the displacement detection resolutions of the displacement sensors 13 and 14 are significantly degraded, i.e. as low as several nm. In the meantime, noise is superimposed on displacement detection signals of the displacement sensors 13 and 14. When the feedback drive circuits 7 and 8 sample these displacement detection signals at a predetermined time, a waveform having crests and troughs of noise are regionally repeated without increasing (or decreasing) the displacement detection signal linearly. When the voltage values of drive signals Dx and Dy are controlled by comparing the displacement detection signal and the scan instruction signal with each other, the piezoelectric unit 4 is finely vibrated and the tip end of the probe 2 and the surface of the sample 1 collide with each other, possibly causing damages to both.
Although the conventional SPM potentially has an excellent capability to obtain image data by a high resolution of its piezoelectric unit, the resolution of the image is degraded to a level of the resolutions of the displacement sensors 13 and 14 when the surface data of the sample 1 is visualized.